Graphitic Carbon Nitride Nanosheets—Nafion as a Methanol Barrier Hybrid Membrane for Direct Methanol Fuel Cells

Sulfonated graphitic carbon nitride (s-GCN) is first realized as an effective bifunctional filler in modifying the Nafion membrane for direct methanol fuel cell (DMFC) application. Carbon nitride with amino (−NH₂) and imino (−NH) functional groups was successfully synthesized through one-step calcination of melamine (C₃H₆N₆) and thereafter surface functionalized to incorporate sulfonic acid (−SO₃H) groups. The proton transfer in the Nafion–s-GCN hybrid membrane is enhanced by the presence of sulfonic acid groups in the filler and also by acid–base interaction between −NH₂ groups present on the GCN with −SO₃H groups of hybrid membrane. The Nafion–s-GCN hybrid membrane with an optimized filler content exhibits a proton conductivity of 183 mS cm–¹, which is about 36% higher in relation to the pristine Nafion membrane. In addition, porous nature of GCN nanosheets in the hybrid membrane enhances movement of hydronium ions while acting as a barrier for methanol molecule. As a result, the Nafion–s-GCN hybrid membranes displayed much lower methanol crossover current density than that of the pristine Nafion membrane. The DMFC comprising Nafion–s-GCN (0.5 wt %) hybrid membrane delivers a peak power density of 125 mW cm–² at 70 °C under ambient pressure, while a peak power density of 65 mW cm–² is realized for pristine Nafion membrane under similar operating conditions.